In an over-sampling A/D conversion circuit as a semiconductor integrated circuit, which is, for example, a ΔΣ-type A/D conversion circuit, a potential selection circuit is used as a D/A conversion circuit for feeding back an A/D conversion output. In a bi-level potential selection circuit, a high-potential terminal and a low-potential terminal are provided as potential output terminals. The high-potential terminal VREF+ is set to 3.0V and the low-potential terminal VREF− is set to 0V, for example. A MOS transistor is provided at an output part from each terminal to the potential output terminal so that a potential is outputted to the output terminal through the MOS transistor in response to a corresponding control signal.
However, in recent years, such a circuit for outputting a potential Is affected largely by a characteristic of a MOS transistor such as a MOSFET used as a switching element in a case that an A/D conversion circuit needs an output of high precision with an increased number of bits. When a ratio of on-resistance/off-resistance of the MOS transistor is not satisfactory, a leak current of the MOS transistor tends to cause error in the output potential in its off-state.
For example, in a circuit configuration that MOS transistors are connected as switching elements between the potential output terminal and the high-potential terminal and the low-potential terminal, one and the other of the MOS transistors are turned on and off, respectively, to output either one of the potentials. When the MOS transistor in the off-state has a leak, the output potential includes an error.
To reduce leak current, it is proposed in U.S. Pat. No. 7,332,941 to connect two MOS transistors in series as switching elements and connect a further MOS transistor to fix an intermediate node of these series-connected MOS transistors to another intermediate potential. This configuration is effective to reduce the leak current. However, it needs a potential generation circuit for generating the intermediate potential and necessarily increases a circuit area.
It is alternatively proposed in JP 2005-268895 A to fix Intermediate nodes of a high potential side and a low potential side to a ground level and a power supply voltage level through MOS transistors, respectively. In this configuration, the MOS transistors, which fix the intermediate nodes, generate leak. This leak causes output error.